1. Field of the Invention
The present invention relates to a nitride semiconductor device such as a light-emitting device, laser, or the like, and more particularly to a nitride semiconductor device capable of increasing carrier injection efficiency and at the same time, capable of preventing reduction of light efficiency due to stress by improving a structure of a lower clad layer of an active layer thereof.
2. Description of the Related Art
Generally, a nitride semiconductor device is a high output power optical device that produces short wavelength light such as blue light, green light, or the like, thus enabling it to produce a variety of colors, and is actively employed as a light emitting diode (LED) and a laser diode (LD). Usually, the nitride semiconductor device is prepared by growing an epitaxial layer on a substrate such as sapphire or SiC using Metal Organic Chemical Vapor Deposition (MOCVD), and the like.
Light efficiency of such a nitride semiconductor device is determined by internal quantum efficiency and light-extracting efficiency. Generally, internal quantum efficiency is the recombination probability between electrons and holes in the active layer and is determined by the structure of the active layer and the quality of the nitride epitaxial layer.
As conventional schemes to improve internal quantum efficiency, there have been proposed doping of Si on a quantum barrier layer (Applied Physics Letters Vol. 73, 1128 (1998)), and improving film quality of the quantum barrier layer by differing growth temperature between a quantum well layer and the quantum barrier layer (Physics Stat. Sol. 9a), 176, 649 (1999)).
Alternatively, a scheme to increase carrier injection efficiency by improving the structure of a clad layer adjacent to the active layer is under study. For example, in accordance with U.S. Pat. No. 5,959,307, by configuring a first clad layer having an energy band gap larger than that of the active layer, a second clad layer having an energy band gap smaller than that of the first clad layer and a third clad layer having an energy band gap larger than that of the second clad layer as a p- or n-type clad layer, sequentially closer to the active layer, electrons injected from the third clad layer were injected by tunneling from the first thin clad layer into the active layer and overflow of carriers from the active layer were prevented by the first clad layer having a high energy band gap. Therefore, even when the temperature of the device rises, an optical device capable of generating high output power without overflow of carriers can be provided.
In order to configure the clad layers satisfying energy band gap conditions required by the above-mentioned reference, Al content is controlled to secure large difference of energy band gap therebetween. For instance, when it is desired to construct the lower clad layer (usually, an n-type clad layer), the first and third clad layers are formed of AlGaN, and the second layer is formed of GaN.
However, when different clad layers as described above were designed, stress due to differences of lattice constants therebetween occurred, and this stress may be applied to the active layer. On the active layer to which the stress is applied, a piezoelectric field is formed and thus a phenomenon occurrs in which a distance between wave functions of electrons and holes increases in the active layer. As a result, this phenomenon causes a reduction of light efficiency of the device.